The present invention pertains to the field of computer driven printer/plotters particularly designed for producing engineering or other large drawings on paper, vellum, film or other printing media which is drawn through the mechanism from a roll thereof. Typically, the medium may have a width from 81/2 inches to as much as 3 or 4 feet or more.
With reference to a rectangular coordinate system, the paper or other printing medium is drawn through the printer in the X direction and a thermal inkjet printer carriage is mounted for movement transversely of the paper in what shall be referred to as the Y direction. A sheet of paper or other printing medium is either manually fed or paper is drawn from a supply roll thereof around a platen roller which may or may not be power driven. Since the printer/plotter apparatus with which the present invention is used employs a thermal inkjet printing head or heads, precise control of the spacing between the print heads and the surface of the medium on which printing is to take place is essential otherwise acceptable print resolution is lost.
When paper or other printing medium is first fed into the printer/plotter, provision must be made to accurately guide it around the drive or platen roller in a direction which is perpendicular to the axis of the platen roller without wrinkling or crimping of the medium upon which printing is to take place. Prior art pinch rollers which pinch the printing medium against the drive or platen roller frequently do not accurately track in the direction of movement of the medium (the X direction) thus causing misalignment and/or wrinkling or tearing of the printing medium. The present invention is directed to the problem of providing a print medium feed and holding arrangement using pinch rollers which may be easily retracted away from the platen roller for feeding of the printing medium into the printer/plotter and which, when in pinching position wherein the pinch rollers are spring biased against the printing medium thus pinching it against the platen roller, are self centering whereby the tracking of the pinch rollers is precisely in the X direction thus preventing misalignment or damage to the printing medium.
Accordingly, the present invention provides a printing media feed and retaining apparatus for a printer/plotter comprising:
a) a pinch roller support member having an arcuate media guide surface thereon, said support member being moveable into and out of a pinching position adjacent a platen roller;
b) said pinch roller support member having a plurality of pinch rollers freely rotatably mounted on at least two spaced parallel axles affixed to said print roller support member; and
c) said pinch roller support member having a support and actuation arm extending therefrom on the side opposite said arcuate media guide surface, said arm being positioned substantially in the middle of a polygon having one each of said pinch rollers at the intersections of the sides thereof.